FIG. 8 is a perspective view illustrating a prior art semiconductor laser including a ridge structure (hereinafter referred to as a ridge type semiconductor laser), and FIGS. 9(a)-9(e) are cross-sectional views illustrating process steps in a method of fabricating the semiconductor laser. FIG. 10 is a diagram illustrating a refractive index profile in a ridge region of the semiconductor laser.
In FIG. 8, reference numeral 1 designates an n type GaAs semiconductor substrate. An n type AlGaAs cladding layer 2 comprising Al.sub.0.5 Ga.sub.0.5 As is disposed on the n type GaAs semiconductor substrate 1. A quantum-well active layer 3 comprising undoped InGaAs is disposed on the n type AlGaAs cladding layer 2. A p type AlGaAs first cladding layer 4 comprising Al.sub.0.5 Ga.sub.0.5 As is disposed on the quantum-well active layer 3. A p type AlGaAs etch stopping layer 5 comprising Al.sub.0.7 Ga.sub.0.3 As is disposed on the p type AlGaAs first cladding layer 4. A p type AlGaAs second cladding layer 6 comprising Al.sub.0.5 Ga.sub.0.5 As and a p type GaAs first contact layer 7 are successively disposed on the p type AlGaAs etch stopping layer 5, and have a stripe-shaped ridge structure. Reference numeral 12 designates a ridge waveguide, and the ridge waveguide 12 has a width Wb in a range of 1.about.1.5 .mu.m at a boundary between the ridge waveguide and the p type AlGaAs etch stopping layer 5. N type AlGaAs current blocking layers 14 comprising Al.sub.0.7 Ga.sub.0.3 As are disposed on the p type AlGaAs etch stopping layer 5 at both sides of the ridge structure comprising the p type AlGaAs second cladding layer 6 and the p type GaAs first contact layer 7. A p type GaAs second contact layer 9 is disposed on the ridge structure and on the n type AlGaAs current blocking layers 14. A p side electrode 10 is disposed on a rear surface of the n type GaAs semiconductor substrate 1, and an n side electrode 11 is disposed on the p type GaAs second contact layer 9.
A description is given of the fabricating method.
Initially, as shown in FIG. 9(a), the n type AlGaAs cladding layer 2, the InGaAs quantum-well active layer 3, the p type AlGaAs first cladding layer 4, the p type AlGaAs etch stopping layer 5, the p type AlGaAs second cladding layer 6, and the p type GaAs first contact layer 7 are successively epitaxially grown on the n type GaAs semiconductor substrate 1, preferably by metal organic chemical vapor deposition (MOCVD).
Next, as shown in FIG. 9(b), a stripe-shaped insulating film 13 comprising Si.sub.3 N.sub.4 or SiO.sub.2 is deposited on the p type GaAs first contact layer 7. The insulating film 13 serves as a mask for ridge etching. In the step of FIG. 9(c), using the insulating film 13 as a mask, the p type AlGaAs second cladding layer 6 and the p type GaAs first contact layer 7 are selectively etched to form a stripe-shaped ridge structure. The selective etching is performed using an etchant, such as a solution of tartaric acid and hydrogen peroxide, that does not etch the p type AlGaAs etch stopping layer 5 but etches the p type AlGaAs second cladding layer 6 and the p type GaAs first contact layer 7. Therefore, the ridge structure comprising the p type AlGaAs second cladding layer 6 and the p type GaAs first contact layer 7 can be formed with good reproducibility.
Thereafter, as shown in FIG. 9(d), the n type AlGaAs current blocking layer 14 is grown on both sides of the ridge structure to bury portions of the p type AlGaAs second cladding layer 6 and the p type GaAs first contact layer 7 which are removed by the etching. Since the insulating film 13 serves as a mask during the crystal growth, the n type AlGaAs current blocking layer 14 is not grown on the ridge structure.
In the step of FIG. 9(e), after removing the insulating film 13 by wet etching or dry etching, the p type GaAs second contact layer 9 is grown on the entire surface. The n side electrode 10 is formed on the n type GaAs semiconductor substrate 1 and the p side electrode 11 is formed on the p type GaAs second contact layer 9, resulting in the semiconductor laser shown in FIG. 8.
A description is given of the operation.
When a voltage is applied across the electrodes so that the p side electrode 11 is plus and the n side electrode 10 is minus, holes are injected into the quantum-well active layer 3 through the p type GaAs second contact layer 9, the p type GaAs first contact layer 7, the p type AlGaAs second cladding layer 6, the p type AlGaAs etch stopping layer 5, and the p type AlGaAs first cladding layer 4 and electrons are injected into the quantum-well active layer 3 through the n type GaAs semiconductor substrate 1 and the n type AlGaAs cladding layer 2. Then, the electrons and holes recombine in the quantum-well active layer 3 and stimulated emission light is generated therein. When the quantity of carriers (electrons and holes) which are injected into the active layer is sufficiently large and light exceeding the waveguide loss is produced, laser oscillation occurs.
In a region in the vicinity of the n type AlGaAs current blocking layer 14 except the stripe-shaped ridge region, pn junctions are formed at the interfaces between the n type AlGaAs current blocking layer 14 and the p type AlGaAs first cladding layer 4 and between the n type AlGaAs current blocking layer 14 and the p type GaAs second contact layer 9. Therefore, even when a voltage is applied so that the p side electrode 11 is plus, the region in the vicinity of the n type AlGaAs current blocking layer 14 is reversely biased because of the p-n-p junction, so that no current flows through this region. That is, the n type AlGaAs current blocking layer 14 blocks current flow. Consequently, a current flows only through the ridge region and is concentrated only in a central portion of the quantum-well active layer 3 just below the ridge region, whereby a current density sufficient to produce laser oscillation is achieved.
A description is given of a waveguide structure for laser light in the prior art semiconductor laser.
Generally, in a semiconductor laser, various structural devices have been used in order to realize a unimodal laser beam having a fundamental transverse mode. More specifically, a semiconductor laser has a waveguide structure comprising a double heterostructure in a direction perpendicular to a pn junction, i.e., in a direction perpendicular to a substrate surface, whereby a laser beam having a fundamental transverse mode is produced stably. Therefore, in the prior art semiconductor laser, since the AlGaAs cladding layers 2, 4, and 6 have respective refractive indices smaller than the refractive index of the InGaAs quantum-well active layer 3, the laser light is guided in the quantum-well active layer 3 having a relatively large refractive index. This is because light has the property of passing through a medium having a large refractive index.
In addition, in a ridge type semiconductor laser, a ridge waveguide has a refractive index profile as shown in FIG. 10 in a direction parallel to the pn junction, i.e., in a direction parallel to the substrate surface, whereby a laser beam having a fundamental transverse mode is produced. Therefore, in the prior art semiconductor laser, since the p type AlGaAs cladding layer 6 in the ridge waveguide 12 has a refractive index larger than that of the n type AlGaAs current blocking layer 14, the laser light is guided along the ridge waveguide 12. Consequently, the horizontal transverse mode that is an important characteristic of the semiconductor laser becomes stable and unimodal.
As described above, the prior art semiconductor laser shown in FIG. 8 guides the light, utilizing the difference in refractive index in the ridge structure. In this semiconductor laser, however, in view of mode control, the waveguide width Wb at the boundary between the ridge waveguide 12 and the etch stopping layer 5 must be in a range of 1.about.1.5 .mu.m. It is probable that a semiconductor laser having a waveguide width Wb larger than 1.5 .mu.m produces higher-order modes higher than or equal to the second-order mode as well as a fundamental mode, and the semiconductor laser producing the higher-order modes shows a nonlinear characteristic called a "kink" in the current-light output characteristic, which adversely affects the laser in practical use. Further, when the semiconductor laser is used to output the laser beam to a fiber, generation of a multimodal laser beam having higher-order modes would exceptionally lower the coupling efficiency between the fiber and the semiconductor laser. Consequently, in order to fabricate this kind of semiconductor laser stably, it is desirable that the waveguide width Wb should be about 1 .mu.m, considering its margin.
However, when the waveguide width Wb is small, current density during operation becomes extremely high, or optical density at the semiconductor laser facet becomes high. Generally, reliability of a semiconductor laser is reduced by internal deterioration and facet destruction. The internal deterioration is caused by an increase of dislocations in an active layer at high current density, and facet destruction is caused by melting of facet portions resulting from high optical density. Therefore, in the prior art semiconductor laser shown in FIG. 8, since the waveguide width Wb is small, i.e., 1.about.1.5 .mu.m, internal deterioration under high current density and facet destruction resulting from high optical density occur, so that reliability of the semiconductor laser is extremely reduced.
In addition, the half-power angular width of the horizontal transverse mode depends on the waveguide width Wb. As the waveguide width Wb is reduced, the half-power angular width varies greatly even when the width Wb varies slightly. Accordingly, the half-power angular width of the horizontal transverse mode varies widely as the waveguide width Wb is reduced, whereby the fabrication yield of the semiconductor laser is reduced. Consequently, in the prior art semiconductor laser shown in FIG. 8, since the waveguide width Wb is small, i.e., 1.about.1.5 .mu.m, the half-power angular width of the horizontal transverse mode varies widely, whereby the fabrication yield of the semiconductor laser is reduced.
Further, the prior art semiconductor laser shown in FIG. 8 includes the current blocking layer 14 comprising Al.sub.0.7 Ga.sub.0.3 As, and has a structure utilizing the difference in refractive index in the ridge structure, i.e., a refractive index type structure. Therefore, the light extending to both sides of the ridge structure is not absorbed, so that it is probable to produce higher-order modes having peaks at the end portions of the ridge waveguide, whereby the coupling efficiency between the semiconductor laser and the optical fiber is lowered.
Patent Application No. Hei. 7-178759 discloses another prior art ridge type semiconductor laser in which a quantum-well active layer emitting laser light having a wavelength of 0.98 .mu.m and a ridge waveguide comprising a p type Al.sub.0.5 Ga.sub.0.5 As cladding layer having a width of 2.about.5 .mu.m are disposed on an n type GaAs semiconductor substrate, and Al.sub.0.7 Ga.sub.0.3 As current blocking layers doped with Er as a metal for absorbing the laser light having a wavelength of 0.98 .mu.m are disposed at both sides of the ridge waveguide.
The above-described prior art semiconductor laser includes a loss guide type structure in which the Er-doped Al.sub.0.7 Ga.sub.0.3 As current blocking layer is used in place of the n type AlGaAs current blocking layer 14 in the prior art semiconductor laser shown in FIG. 8 and the laser light is absorbed by this Er-doped AlGaAs current blocking layer. More specifically, in this prior art semiconductor laser, Er ions in the current blocking layer absorb the laser light having a wavelength of 0.98 .mu.m which is emitted from the quantum-well active layer just below the ridge waveguide. The absorption of the laser light by the current blocking layer is promoted more at the end portions than at the center portion of the ridge waveguide, whereby gains of higher-order modes having the peaks at the end portions of the ridge waveguide are reduced. Consequently, according to this prior art semiconductor laser, even when the waveguide width is 2.about.5 .mu.m, a unimodal laser beam having a fundamental transverse mode is produced stably.
FIG. 11 is a cross-sectional view illustrating another prior art semiconductor laser disclosed in Japanese Published Patent Application No. Hei. 5-21902. In the figure, reference numeral 101 designates an n type GaAs substrate. An Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 102, a double quantum-well active layer 103, a p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 104, and a p type GaAs ohmic contact layer 105 are successively disposed on the n type GaAs substrate 101. The p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 104 and the p type GaAs ohmic contact layer 105 have a stripe-shaped ridge structure 106 produced by selective etching. N type GaAs current blocking layers 111 doped with silicon (Si) are disposed so as to bury portions of the upper cladding layer 104 and the ohmic contact layer 105 which are removed by the etching. An n side electrode 108 is disposed on a rear surface of the GaAs substrate 101 and a p side electrode 109 is disposed on the ridge structure and on the current blocking layers 111. The active layer 103 has a quantum-well structure comprising an Al.sub.0.3 Ga.sub.0.7 As barrier layer 103c, two GaAs quantum-well layers 103d, and two Al.sub.0.3 Ga.sub.0.7 As guiding layers 103b. The AlGaAs barrier layer 103c is sandwiched between the two GaAs quantum-well layers 103d, and further, the GaAs quantum-well layers 103d are sandwiched between the two AlGaAs Guiding layers 103b.
In the prior art semiconductor laser shown in figure 11, the n type GaAs current blocking layer 111 is doped with Si to a high concentration, the current blocking layer 111 has an n type carrier concentration equal to or larger than 6.times.10.sup.18 cm.sup.-3, and the current blocking layer 111 absorbs light having a wavelength equal to or larger than 900 nm which travels from the active layer 103 to the n type GaAs current blocking layer 111 through the upper cladding layer 104 during laser oscillation. More specifically, according to this prior art semiconductor laser shown in FIG. 11, since a broad deep level extending over a 900.about.1000 nm band is formed in the n type GaAs current blocking layer 111, the current blocking layer 111 absorbs the light having a wavelength equal to or larger than 900 nm which travels from both sides of the light emitting portion to the current blocking layer 111 through the upper cladding layer 104, whereby generation of higher-order modes is suppressed and a fundamental mode is produced.
In the prior art semiconductor laser disclosed by Patent Application No. Hei. 7-178759, however, when the current blocking layer is doped with Er, the absorption peak is shifted from 0.98 .mu.m due the narrow absorption band; Er ions have an absorption band for light having a wavelength of 0.98 .mu.m. Therefore, it is impossible to suppress the generation of the higher-order modes reliably.
In the prior art semiconductor laser shown in figure 11, the broad deep level is formed in the vicinity of a 900 .about.1000 nm band in the n type GaAs current blocking layer 111 which is doped with Si to a high concentration. However, due to insufficient light absorption, the generation of the higher-order modes cannot be suppressed reliably.